Science Questions

Does dark matter have structure?

Sat, 14th Aug 2010

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Leo Vela asked:

Is dark matter simply a framework around which matter affixes itself to, or does dark matter have its own independent structures - meaning is there a dark universe with dark galaxies, and dark planets, and maybe even dark lifeforms?


Thank You,


Leo Vela

Miami Beach, Florida


We put this question to Dr Andrew Pontzen:

Andrew -   Well firstly, normal matter in the universe is really clumped together.  Itís not evenly spread out through the universe, itís clumped, for instance into galaxies.  Galaxies are collections of around 100 billion stars, and they're relatively compact, and there's a lot of relatively empty space between each different galaxy.  Now, dark matter certainly does clump together in that sense.  We know that for sure, from observations of galaxies.  

This was in fact the original evidence for dark matter, looking at the way that material like stars and gases moves around in galaxies, and inferring from that strength of the gravitational field in galaxies, and from that inferring how much stuff was there, and thatís how we knew that there had to be dark matter.

In fact, because thereís so much more dark matter than normal matter in the universe - thereís around five times more dark matter than normal directly visible matter - its clumping is incredibly important in terms of determining the kind of structures that form in the visible universe, and the existence of galaxies effectively owes itself to dark matter.  So in that sense, there are structures in the dark matter that are similar to the ones you see directly in the normal matter.

On another level though, we don't really know what dark matter is, and so, when weíre talking about dark matter, we tend to be modelling it subject to some simple assumptions about what itís doing, and you get the best results for the evolution of the universe, matching what we see in the real universe, when you model the dark matter as completely non-interacting, except through gravity.  So, other than the gravitational force which it exerts, and which itís also subject to, itís not subject to any other forces, for instance, the electromagnetic force which normal matter is subject to.

In fact, the really interesting structures in normal matter arise through things like the electromagnetic force in all of chemistry for instance, and therefore, life really arises through forces like the electromagnetic force.  And for that reason, the evidence at the moment would suggest that you can't have really complicated structures that will be required to create what you might describe as dark life forms.  So, most likely, thereís nothing quite that interesting going on in the dark sector, but until we really know what it is, we can't say for absolute definite.


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Leo Vela asked the Naked Scientists: Is dark matter simply a framework around which matter affixes itself to or does dark matter have its own independent structures, meaning is there a dark universe with dark galaxies, and dark planets, and maybe even dark lifeforms? Thank You, Leo Vela Miami Beach, Florida What do you think? Leo Vela , Thu, 17th Jun 2010

I am not sure of the others but you can find dark lifeforms in the the souls of man right here on earth.
Just recently stumbled over this link.
There is some info on the research relating to an error in calculating the big bang theory

It is to bad because it really captured my imagination. I was thinking that dark matter was full of least energetic photons just waiting there to exchange energies. That is me dreaming. tommya300, Thu, 17th Jun 2010

Dark matter does have a weak structure on the scale of galaxies and galaxy clusters and it has a very slight change of density associated with gravitating masses like stars and planets but it cannot have much fine structure because it interacts only by gravity (or it would have easily been detected) and the particles cannot loose energy by collisions (other than by creating very weak gravitational waves) and particles do not stick together like atoms do. Soul Surfer, Fri, 18th Jun 2010

I agree with SS. The models of the distribution of dark matter necessary to give the apparent distribution of mass and velocities of normal matter in galaxies (and galaxy clusters) suggest it only interacts with matter very weakly. The distribution seems to be (very roughly) spherical even with galaxies which have settled into discs, and extends well beyond the galaxy's edge. This would suggest it only interacts with itself very weakly too. It is therefore unlikely that there would be much detail in any "structure" to dark matter. The forming of anything complex with dark matter would be an idea that may even stretch the writers of Star Trek. graham.d, Fri, 18th Jun 2010

Hi SS and all
Would you consider the Oort cloud as dark matter?

CliveS acsinuk, Fri, 18th Jun 2010

No, at least not with what is usually defined as "dark matter". The Oort cloud is thought to be conventional matter and the source of comets - definitely normal matter (ice, methane, etc.). graham.d, Fri, 18th Jun 2010

I've just posted this in the other current thread on DM/DE but it applies here too:

LeeE, Sun, 20th Jun 2010

What you said is true, Lee, but then that is ofetn the way scientific progress is made. Anti-matter, and in particular the positron, was theoretically proposed long before its discovery. graham.d, Sun, 20th Jun 2010

In a vacuum of space, having a hypothetical glass, half full or half empty, either portion has to have something occuping that other particular area. Since math does not explain absolutely, things can be piled higher and deeper, fudge factors are acceptable if the majority vote of the committee can agree.

LeeE has made a good point. tommya300, Sun, 20th Jun 2010

There's my take on the first question in this thread in the next Naked Astronomy podcast, but it more-or-less agrees with Soul Surfer's logic. (I wrote it independently, honest!)

As to not requiring dark energy/dark matter any more, I've written a critical evaluation of that claim over in the other thread.

Monthly astronomy podcasts -- Andrew P, Sun, 20th Jun 2010

That one was really interesting tommya300 :)

"Clara Moskowitz, senior writer at wrote:Instead of using Jupiter as a calibration source, the way the WMAP team did, Shanks and Sawangwit used distant astronomical objects in the WMAP data itself that were emitting radio light.

"When we checked radio sources in the WMAP background, we found more smoothing than the WMAP team expected," Shanks told "That would have big implications for cosmology if we were proven right."

If this smoothing error is larger than thought, it could indicate that fluctuations measured in the intensity of the CMB radiation are actually smaller than they originally appeared. The size of these fluctuations is a key parameter used to support the existence of dark matter and dark energy.

With smaller ripples, there would be no need to invoke exotic concepts like dark matter and dark energy to explain the CMB observations, Shanks said. The researchers will report their findings in an upcoming issue of the journal Monthly Notices of the Royal Astronomical Society."

The paper published is here.
And this one have a discussion on it too, explaining the idea in some detail. yor_on, Mon, 28th Jun 2010

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